FIG. 1 is a sectional view showing an example of a configuration of the conventional CMUT (Capacitive Micromachined Ultrasonic Transducer)-type ultrasonic transducer. The conventional CMUT-type ultrasonic transducer comprises a vibrating membrane 100 which sends and receives an ultrasonic wave, and a vibrating membrane supporting section 101 which is provided on one face of a substrate 104 and supports the vibrating membrane 100 so that the vibrating membrane 100 faces the substrate 104. A membrane-side electrode 102 is formed on the vibrating membrane 100, a substrate-side electrode 103 is formed on the substrate 104, and the membrane-side electrode 102 and the substrate-side electrode 103 are arranged opposite to each other.
In such a CMUT-type ultrasonic transducer, the vibrating membrane 100 and the membrane-side electrode 102 vibrate by the received ultrasonic wave (sound pressure). In this case, the CMUT-type ultrasonic transducer obtains an electric signal for the received ultrasonic wave based on a capacitance change between the membrane-side electrode 102 and the substrate-side electrode 103, or applies DC voltage and AC voltage between the membrane-side electrode 102 and the substrate-side electrode 103, thereby causing the vibrating membrane 100 to vibrate and transmitting an ultrasonic wave. The CMUT-type ultrasonic transducer has a frequency response characteristic such as a wideband, a high sensitivity, and the like.
For example, the conventional CMUT-type ultrasonic transducer and a method for manufacturing the same are disclosed in “Capacitive Micromachined Ultrasonic Transducers: Theory and Technology”, JOURNAL OF AEROSPACE ENGINEERING, USA, April, VOL. 16, NO. 2, p. 76-84. In detail, in the CMUT-type ultrasonic transducer, a nitride layer for protecting the substrate is formed on a silicon substrate in a case of later-described wet etching, and the so-called sacrificial layer containing polycrystalline silicon is vapor-deposited on the nitride layer. Then, both a vibrating membrane containing a nitride layer and a vibrating membrane supporting section are vapor-deposited on the above-mentioned sacrificial layer, and a hole for removing the sacrificial layer is formed on the vibrating membrane. Via the hole, an etching solution is poured in and the sacrificial layer is removed by wet etching. Subsequently, the hole is filled, a membrane-side electrode is vapor-deposited on the vibrating membrane, and then a protective layer is formed on the membrane-side electrode to produce the CMUT-type ultrasonic transducer.